1. Field of the Invention
The present invention relates to a measuring apparatus for measuring a cutting blade width of a cutting machine, for polishing the cutting blade as needed, and for automatically cutting a sheet material such as a dishcloth by computer control.
2. Description of the Related Art
Hitherto, automatic computer controlled cutting machines have been employed for cutting flexible sheets of material such as textile fabric, knitted fabric, paper, plastic, and leather sheet. A cutting blade for cutting sheet materials becomes dull as cutting is continued, and hence it is necessary to frequently polish/sharpen the cutting blade. Therefore, the cutting machine is required to have a cutting blade polishing apparatus in order to continue cutting while polishing.
The cutting blade is made of a hard metal material such as stainless steel and high speed tool steel. Nevertheless, the blade edge gradually wears due to frequent polishing. Consequently, it is necessary to provide the control apparatus in prior art cutting machines with data to compensate for the change in blade width. The data is obtained by an operator with a measuring tool, such as calipers and a gauge, so that the wear amount of the blade edge can be compensated and accurate cutting can be carried out along a cutting line.
In the prior art, in order to measure the blade width with a gauge, it is necessary to waste an inordinate amount of time, for example, removing respective parts enveloping the periphery of the cutting blade, which results in decreased productivity.
A typical prior art cutting machine, having a polishing apparatus is disclosed in Japanese Examined Patent Publication JP-B2 57-14957 (1982) corresponding to U.S. Pat. No. 4,133,233 and Japanese Unexamined Patent Publication JP-A 57-194865 (1982). In the prior art machine, a polishing mechanism is provided near the cutting blade, and it is designed to allow polishing during the down time between cutting operations or during a cutting operation.
Further, as disclosed in the Japanese Unexamined Patent Publication JP-A 54-96888 (1979), corresponding to the Japanese Examined Patent Publication JP-B2 57-14957 (1982), a compensating control apparatus automatically compensates for a loss of cutting blade width caused by the grindstone. By means of the control apparatus, the changes in the blade width are compensated on the basis of the wear amounts obtained from values recorded by a polishing cycle counter.
In the prior art, the blade width is not actually measured, rather it is estimated on the basis of the wear amounts of the cutting blade which are determined from the number of times the blade is polished and the amount of polishing time. Accordingly, depending on the working condition of the grindstone, errors in estimating the blade width occur, and accurate cutting according to a pattern cannot be achieved due to accumulation of the errors. Further, in the case where it is necessary to replace the cutting blade, the wear data regarding the previous cutting blade is stored regardless of the specific kind of cutting blade, so that it is necessary to modify the data to correspond with the replacement cutting blade. This modification requires excessive time and labor. Similar problems occur when a cutting blade replacement is required due to a malfunction during the cutting operation.
Although a mechanism for polishing the cutting blade is disclosed in the prior art, the concept of controlling the amount of polishing which is directly related to the amount of cutting blade wear is not disclosed. However, insufficient polishing leads to insufficient cutting sharpness of the cutting blade, and a favorable cutting result cannot be obtained. On the other hand, excessive polishing leads to early wearing and a shortened life of the cutting blade and grindstone.
FIG. 12 illustrates a relationship between the polishing frequency and the polishing amount, i.e. change in width of a cutting blade. When polishing is repeated under certain conditions, the relationship between the polishing frequency and amount did not represent a straight line as indicated by the twin dot chain line, but instead represented an upward convex curve as indicated by the solid line. When polished more than 2000 times, it is estimated that the polishing amount will be smaller than that in the line indicated by the twin dot chain line. Such a change in polishing amount is caused by clogging or wearing of the grindstone resulting in reduced polishing performance.
If an automatic polishing mechanism is incorporated in a cutting machine as in the disclosed prior art, it is generally set to polish slightly excessively in consideration of the lowering performance of the grindstone. Accordingly, blade and grindstone life is shortened because the wearing of the cutting blade and grindstone is accelerated. Additionally, since lowering of the polishing performance of grindstone cannot be detected accurately, the timing of replacement or inspection of the grindstone is often determined on the basis of operator experience, and therefore it is difficult to keep the cutting blade in an optimal cutting condition without shortening blade life.
FIGS. 13A, 13B show general relationships between polishing time and polishing amount. FIG. 13A shows the relationship between the cumulative polishing time and amount of grindstone, and FIG. 13B shows the relationship between the polishing time and individual amount of cutting blade. As shown in FIGS. 13A, as the cumulative polishing time of the grindstone increases, the polishing amount indicated by the solid line comes to deviate from the straight line indicated by the twin dot chain line. Accordingly, in the initial state of the grindstone in the period of t0 to t1, the polishing performance is as indicated by a line La in FIG. 13B, but in the terminal state of the grindstone in the period of t2 to t3, the polishing performance is as indicated by line Lb. When the polishing performance is indicated by the line Lb, it takes a longer time to get the same polishing amount of the cutting blade than when the polishing performance is indicated by the line La. When operating at line Lb the efficiency is low, and the useful grindstone life is over. That is, for one grindstone, when the polishing amount reaches A, it has reached the end of its useful life. As shown in FIG. 13B, the time it takes to reach the end of the cutting blade useful life is much shorter than the life of the grindstone. Thus, the cumulative polishing time of the grindstone and the polishing amount of the cutting blade vary substantially with the polishing performance of the grindstone.